1. Field of the Invention
The present invention relates to a microphone apparatus. More particularly, the present invention relates to a housing and mounting system for a directional microphone that eliminates extraneous reflecting surfaces, increases the front-to-back signal strength and improves the overall gain and frequency response of the microphone.
2. Description of Related Art
In 1933, J. Weinberger, H. F. Olson and F. Massa, J. Acoust. Soc. Amer., 5,139 (1933), it was shown how to combine two microphones with omnidirectional and figure-of-eight directivity characteristic into single sound receiver with a polar diagram of cardioid shape. The ideal cardioid characteristics is shown in the polar plot of FIG. 1. In 1935, von Braunmuhl and Weber, Hochfrequenztechnik u. Elektroakustik, 46, 187-192 (1935), described methods designed to modify the hitherto purely pressure type condenser microphone into a pressure-gradient type with cardioid directional characteristic. The microphone built as shown in FIG. 2 is sensitive to pressure gradient, has figure-of-eight directivity characteristic. The structure of the microphone shown in FIG. 2 has a single diaphragm 1 adjacent to an electrode 2. Alternatively, the microphone structure of FIG. 2 can be provided by fixing two equal diaphragms, 3 and 4, to both sides of the perforated fixed middle electrode 5, as shown in FIG. 3.
In an effort to achieve smooth frequency response, fast impulse response, and good front-to-back ratio further developments in microphone technology were made. A. Dauger and C. F. Swisher, A Self-contained Condenser Microphone with Improved Transient Response, Presented Apr. 29, 1968 at 34.sup.th Convention of AES, Los-Angeles, described a single-diaphragm microphone element design utilizing elaborate acoustically resistive delay path behind of the back electrode. FIG. 4 depicts such a microphone structure having a single diaphragm 6, an electrode 7 and resistive delay path material 8.
The operative characteristics of the microphone of FIG. 4 is illustrated in FIG. 5. As shown in the first half of FIG. 5, consider a sound wave coming from the front direction. The wave can be though of as splitting into two parts upon reaching the microphone. Part A reaches the diaphragm 6 directly, and pushes downward on it. Part B goes around to the back and reaches the surface of the acoustical resistive delay network 8 at some time later than Part A reached the diaphragm surface 6. Part B then passes through the network 8, which causes the wave to arrive at the bottom of the diaphragm 6 pushing up on it at a later time than when part A pushed down on it. As a result there is considerable phase difference and hence pressure difference on the diaphragm 6. The diaphragm 6 moves and a signal is generated.
Consider now waves coming from the back, depicted in the second half of FIG. 5. When part A reaches the surface of the delay path 8, part B starts to go around to the front. Part B reaches the front of the diaphragm 6 and pushes down on it some time later. Meanwhile Part A is moving through the delay path 8. If the parameters of the path are chosen properly, Part A reaches the back side of the diaphragm 6 and pushes up on it at the same time part B is pushing down. The diaphragm 6 does not move and no signal results.
Of course, parameters of the delay 8 must be chosen very carefully to provide adequate phase shift for all audio band frequencies. Unavoidable problems also arise at very high frequencies where wavelengths become comparable to microphone element dimensions, which leads to additional phase shift, thereby decreasing the front-to-back ratio. In order to cope with this, the size of the microphone element is made as small as practicable.
In most conventional cardioid microphones the space around and behind the microphone element gets little or no careful acoustical design consideration. The microphone element is usually mounted some distance from the body and has a huge cage-like structure around it, as shown in FIG. 6. Disadvantageously, as a result of the inattention to the details of the housing structure, a large number of reflections (e.g., A' and B') result in such a structure as FIG. 6. These reflections have different arrival times, which causes the phase pattern to be smeared. These reflections lead to peaks and notches on the frequency response, as shown in FIG. 10, which are very audible as sound coloration, and deterioration of front-to-back ratio. FIG. 10 depicts a 0.degree. incident frequency response, a 90.degree. incident frequency response and a 180.degree. incident frequency response, where the incident response is with respect to an axis taken perpendicular to a diaphragm of the prior art directional microphones. The peaks and notches shown in FIG. 10 are largely due to rear signal reflections within the housing structure, which degrades the front-to-back signal strength as well as degrading the overall gain of the microphone. What is worse, these sharp peaks and notches on the off-axis frequency response result in positive acoustic feedback when used in sound reinforcement applications.
In another approach in the prior art to provide a directional microphone structure, Bartlett (U.S. Pat. No. 4,694,499) discloses a directional microphone having an acoustic damping washer positioned adjacent the microphone rear entry. The washer is generally a doughnut-shaped element formed of sound absorbing material and positioned around the rear sound entry port of a directional microphone. The washer is so positioned in an effort to reduce reflections of front-arriving sound and absorb and cancel high-frequency sound which approach the rear of the transducer (microphone). However, Bartlett fails to consider the housing structure around the rear of the microphone, which can lead to extraneous reflecting waves and thus, a degradation of the overall frequency response, as described above. Moreover, Bartlett fails to consider the cumulative affect of the reflected signals within the housing structure that cannot be entirely canceled, thus decreasing the front-to-back signal strength.
Unfortunately, none of the aforesaid directional microphone systems disclose a structure that eliminates extraneous reflecting surfaces within the housing structure, increases the front-to-back signal strength and improves the overall gain (e.g., low frequency response) of the microphone. This is largely due to the failure in the prior art to provide an effective system that cancels virtually all rear signals, thereby approaching ideal cardioid response characteristics.